<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <!-- 代码不全别看了 -->
    <p>02</p>
    <button id="btn">btn</button>
    <a href="5.BOM.html">03</a>
    <script>
        var btn = document.getElementById()
    </script>
</body>
</html>